1. Field of the Invention
The present invention relates to a semiconductor laser and a method for manufacturing the same.
2. Description of the Prior Art
The operational principle of semiconductor lasers and the construction of a conventional semiconductor laser will be described.
Semiconductor lasers are made of a compound semiconductor such as GaAs or InP which has a direct transition-type energy band structure. As voltage is forwardly applied to a p-n junction of such a semiconductor, current flows through the semiconductor. This current flow causes electrons in the n-type region and holes in the p-type region to flow toward the opposite regions to recombine together, and thus, to emit light.
At a small quantity of current flow, the recombination of electrons and holes are achieved irregularly. As a result, induced emission of light required for operating the semiconductor laser is not achieved since resultant optical waves have no correlation with one another. At a large quantity of current flow, however, an inverted electron distribution is formed near the p-n junction, as shown in FIG. 1. The inverted electron distribution means that more electrons at a higher energy level are distributed over a lower energy level. At such an inverted electron distribution, light emission is induced by virtue of the electron-hole recombination. In this case, the region at which the induced emission of light caused by the inverted electron distribution is called an active region or a gain region.
As a gain larger than a loss of a resonator is generated upon an increase in applied current, the laser is oscillated. The current providing the gain required for oscillating the laser is called a threshold current which is an important factor for determining the performance and the application condition of a semiconductor laser.
In a semiconductor laser, the resonator for obtaining the oscillation of optical waves uses mainly the crystal sectional surfaces (namely, the surfaces of cleavage) of the semiconductor itself.
An efficient structure capable of reducing the threshold current in the semiconductor laser is described below. If the recombination of carriers (electrons and holes) occurs at a region of no inverted electron distribution, that is, out of a gain region, the light emitted due to the recombination cannot contribute to the induced emission.
In such a double hetero (DH) structure, an active layer (GaAs) having a small energy gap is interposed between clad layers (GaAlAs) having a large energy gap. An example of a semiconductor laser having such a DH structure is a simple diode in which its one hetero-junction is a p-n junction. As current flows forward through the diode, the holes in the n-type clad layer flow into and are then implanted in the active region.
Since the active layer has a small band gap as shown in FIG. 2, the clad layers disposed at opposite surfaces of the active layer form energy barriers which function to restrain implanted carriers in the active region. Accordingly, the density of carriers in the thin active region is very high and the recombination of carriers for emitting light is mainly achieved in the active layer.
The refractive index of GaAs constituting the active layer is higher than that of GaAlAs constituting the clad layers. Light concentrates in a region having a large refractive index. Accordingly, in the DH structure, light is focused on the active layer. Hence, densities of carriers and optical waves in the active layer are very high, thereby enabling the threshold current to be reduced.
Furthermore, the threshold current can be lowered further by restraining the carriers and the optical waves in the narrow active region in a direction perpendicular to the active layer.
A metal electrode having a narrow strip shape is formed as shown in FIG. 2 to control the current flow. As shown in FIG. 2, opposite side surfaces of the metal electrode are formed roughly to prevent light concentration. Such a structure is called a strip-type hetero structure. It is often called a gain transmission-type structure since light is guided to a gain region in which the density of carriers is high.
In addition, there is a method for restraining optical waves in a direction parallel to the active layer. FIG. 3 shows a buried type hetero structure which is commonly used in communication lasers. As shown in FIG. 3, the structure has a shape in which a GaAs active layer is surrounded at its opposite sides by n-type GaAlAs layers.
As above-mentioned, the GaAs layer forms a waveguide path since it is surrounded at its upper, lower, left and right portions by the GaAlAs layers having a refractive index lower than that of the GaAs layer.
As shown in FIG. 3, opposite side surfaces of the waveguide are formed roughly to prevent light concentration. This type of waveguide is called the refractive index waveguide.
Such a DH structure has an advantage of having a low threshold current. Also, it has a stable oscillation transverse mode characteristic and is advantageous for communication and information processing applications.
Now, problems encountered in the conventional semiconductor lasers shown in FIGS. 2 and 3 will be described in conjunction with semiconductor laser devices which are equipped with the conventional semiconductor lasers and which are shown in FIGS. 4 and 5, respectively.
First, the semiconductor laser device shown in FIG. 4 will be described. In manufacturing the semiconductor laser device, a semiconductor laser chip is attached to one side portion of a cylindrical heat sink plate, as shown in FIG. 4. The semiconductor laser chip is coupled with external elements, such as a light receiving element and an optical communication cable, by means of a grooved block separately provided at one side portion of the semiconductor laser. The contact portions between the semiconductor laser and the block are molded for preventing laser beams from escaping.
However, it is very difficult to couple the semiconductor laser chip with the optical cable by means of the block. Furthermore, the semiconductor laser device shown in FIG. 4 can process only one signal from a single line and has a relatively large size. As a result, it is impossible to integrate a plurality of semiconductor lasers, having the structure of FIG. 4, in manufacturing a semiconductor laser device capable of processing signals from a plurality of lines, simultaneously.
On the other hand, a considerable loss of laser beams occurs at bent portions of the semiconductor laser device. Such a considerable loss of laser beams occurs at the block, since it is impossible to align the center of semiconductor laser chip with the core of the optical cable by the block. As a result, an accurate signal transmission cannot be achieved.
In addition, the structure of FIG. 4 is not economical because the block is needed to couple the semiconductor laser chip with external elements.
Second, the semiconductor laser device shown in FIG. 5 will be described. In manufacturing the illustrated semiconductor laser device, a semiconductor laser chip is formed on a substrate. Then, a guide cavity is formed on the substrate. In the guide cavity, the semiconductor laser chip is coupled with an optical cable. The optical cable and the substrate disposed in the guide cavity is molded with an epoxy resin material to prevent laser beam losses.
In this structure, however, one substrate accommodates only a single semiconductor laser chip which processes a signal from a single line, similar to the structure of FIG. 4. As a result, it is impossible to form a plurality of semiconductor laser chips on a single substrate. This makes mass production of semiconductor lasers difficult.
Similar to the case of FIG. 4, it is impossible to integrate a plurality of semiconductor lasers having the structure of FIG. 5 in manufacturing a semiconductor laser device capable of processing signals from a plurality of lines, simultaneously.